The present invention relates to a technology of monitoring an optical signal in an optical transmission network.
With an increase in communication traffic, optical networks based on a wavelength multiplexing technology have increasingly been built up in a long-haul region, a metro region and an access region, respectively.
In the optical transmission network, Optical Add/Drop Multiplexer (OADM) equipment capable of relaying (Through), transmitting (Add) or terminating (Drop) signals on a wavelength basis of the light is required for building up a more flexible network.
The OADM equipment, a terminal station device and a relay device monitor WDM (Wavelength Division Multiplexing) signals and make a level adjustment of each of the wavelengths in order to transmit the signals at a long distance.
FIG. 1 shows an example of a ring network using the OADMs.
OADM equipment 91, to which the WDM signals are inputted, can drop, add and through let a desired wavelength of the WDM signals.
For example, in the case of dropping the signal, the OADM equipment 91 demultiplexes the signal branched off by an optical coupler and optically receives a desired channel (wavelength). Further, the OADM equipment 91 demultiplexes (DEMUX) (demultiplexing of wavelengths) the WDM signals according to the wavelengths, and inputs the demultiplexed signals to one input of an optical switch. Then, the OADM equipment 91 inputs an add signal to the other input of the optical switch. Subsequently, the OADM equipment 91 outputs the signal selectively on a wavelength-by-wavelength basis by transmitting the signal to a next OADM equipment in a way that lets a through-signal or the add signal through the optical switch.
Then, after passing through a Variable Optical Attenuator (VOA), the MUX (multiplexer) multiplexes the wavelengths of the signals of individual channels, and the thus-multiplexed signals are transferred as WDM signals to a posterior node. At this time, the OADM equipment 91 monitors the wavelength-multiplexed signals with an optical channel monitor (OCM), and controls the VOA so that each wavelength comes to a target level.
Moreover, optical amplifiers 92 are properly provided at anterior-and-posterior stages of the OADM equipment 91, thus scheming to elongate a transmission distance by compensating a loss of the optical level of the signal between the OADM equipments 91 and an insertion loss of optical components of the OADM equipment 91.
Further, in the network RN where the arbitrary OADM equipment 91 thus adds/drops the signal light, the signals transmitted through within the network dynamically change in terms of the number of wavelengths, and hence the optical amplifier 92 performs gain constant control (Automatic Gain Control (AGC)) to retain output optical power of each wavelength at a fixed level with respect to fluctuations in the wavelengths.
Therefore, when the WDM signals pass through the optical amplifiers 92 at several stages, optical noises based on Amplified Spontaneous Emission light (ASE light) are accumulated at unused channels, which might affect the gain control in the optical amplifier 92.
Such being the case, in the OADM equipment 91 and in the variable optical attenuator (VOA) within a terminal station device, an Optical Channel Monitor (OCM) monitors each channel (wavelength), and, if the monitored light is not the signal light but the ASE light, the VOA on a channel basis shuts down (increases an attenuation quantity), thus conducting the control so as not to transmit the ASE light to a downlink node.
At this time, it is determined based on a difference between spectral line shapes whether the monitored light is the signal light or the ASE light. For example, as illustrated in FIG. 2, if being the signal light, the signal light takes a shape having a peak that is narrow in width and large in height at a specified frequency, and, whereas if being the ASE light, the ASE light takes a shape having the same large width as a transmission bandwidth of a filter device.
Moreover, a technology disclosed in, e.g., the following Patent document 1 is given as the prior art related to the invention of the present application.
[Patent document 1] Japanese Patent Laid-Open Publication No. 8-15012